Mystery Bearings???

[Bill S.]

In the process of doing a rear main seal job on a small block 68 we discovered
the main bearing and one rod bearing to have a copper ring like band running through it. Several engine builders were present and we all agreed --------------- what the heck is going on here.......... see picture
we only pulled one rod in addition to the main and MOST of the bearing surfaces appeared this way
Thanks
Bill




Corvette Hacks

[Dick W.]

Re: Mystery Bearings???

Appears to have had a considerable of trash run thru it. Also looks like either showing fatique of the babbit overlay or has gone a considerable period of time without oil change. Acid etching.

[Joe L.]

Re: Mystery Bearings???

Bill------

My guess would be a severely worn "tri-metal" bearing. Original GM engine bearings were premium aluminum over steel shell. No copper was used in the construction of these bearings. These bearings will usually be marked "GMM" on the outer shell.

Many aftermarket bearings use a "tri-metal" construction. In this construction a thick copper-lead alloy overlays the steel shell. Overlaying this is a VERY thin nickel layer (called a nickel dam). Finally, a relatively thin layer of babbit material overlays the nickel dam and forms the actual bearing surface.

In this case, the babbit and nickel dam layers have worn through in the center exposing the copper-lead alloy layer.

Tri-metal bearings were not used by GM for Chevrolet engines in the 50's through, at least, the mid 90's. So, I expect that these are aftermarket bearings like Clevite 77, Michigan 77, or Federal-Mogul "CP" series.

Please inspect the rear of the bearing shell and report whatever markings you might find.

[William C.]

Re: Mystery Bearings???

I believe the "Moraine 400" series used in the SHP engines were tri-metal.

[Duke W.]

Re: Mystery Bearings???

No, the Moraine 400 and 420 are aluminum. The M400s will take higher loading and have greater fatigue resistance than trimetal, but have less embeddibility. As long as oil change intervals are reasonably with good filtration, they will last virtually forever.

Like Joe said, those bearings in the photo look like they have been severely gouged by debris in the oil.

What does the journal look like?

Duke

[John D.]

Re: Mystery Bearings???

Journals, both main and for rod looked fine. No gouging/scoring at all.

Given the above, what should be done from here?

[Chris Patrick]

Re: Mystery Bearings???

Polish the crank, change bearings, check clearances (keep them around .0025), inspect the rod for stretch and roundness of opening, inspect the oil pump for excessive clearance, and quit doing hard downshifts. Beside the junk, poor oil, and lack of frequent oil changes, you appear to have heavy loading on the bottom of the bearing which happens in extreme engine braking (downshifting).

[Duke W.]

Re: Mystery Bearings???

Normal overhaul procedure. Have the block thoroughly cleaned including oil galleries, and check main bearing saddle alignment. Check crankshaft for dimensional confomity and straightness, and have it polished. Inspect the oil pump for wear or replace it. I recommend nominal main bearing clearances of .0015" on the mains and .002" on the rods for a road engine.

I doubt if high speed downshifting has anything to do with the damage. The bearings have plenty of load capacity, and the oiling system is very reliable. Bearing damage is usually an indication of debris in the oil, oil starvation, or dimensional non-conformities.

Upon assembly use Plastigage to check bearing clearances. After the main cap bolts are torqued, the crankshaft should spin freely with very little friction and no sticky spots.

Duke

[Chris Patrick]

Re: Mystery Bearings???

Duke, if you look at that rod bearing, you see the greatest wear is at the bottom. A rod normally has the extreme stress on the upper bearing from pushing down on the crank, not pulling up on the crank.

You only get that pulling up with the piston resisting the crank rotation.

One or two times downshifting isn't going to do it. Heck, most of us will never get enough. So it needs more factors.

Low oil pressure, either by a worn pump (reason I said check pump for clearance), excessive bearing clearance, or oil starvation (G loading of oil reservoir)

Dirty oil - long oil change periods, poor filter, clogged filter, open oil bypass.

The rods were probably going first, and the mains are a result of junk going back through the system if it is from extreme loading.

If the mains were going first, then the oil filtering system is shot and needs a complete overhaul.

[Chuck S.]

Re: Mystery Bearings???

Because of the perfect symmetry of the babbit void on the shell, and it's narrow linear nature, I suspect something else besides negligence contributed here. Maybe those "dimensional non-conformities", or some kind of manufacturing quality issue also contributed.

[Duke W.]

Re: Mystery Bearings???

The highest load on the rod and rod bearing is during the inlet and exhaust stroke when there is no cylinder pressure to offset inertia force, and this loading is a little higher on overrun when the cylinder is below atmospheric pressure during the inlet stroke, but the difference is only about 125 pounds compared to a total load of several thousand pounds at high revs.

Thus I do not consider "high speed downshifting" to be an issue. At 115K miles my '63 L-76 showed essentially no wear on the crank or M400 bearings, and these miles included about a 1000 miles of race track hot-lapping with plenty of high speed downshifts - both on the track and street. High speed downshifts usually extract a toll on transmission synchronizers unless one double clutches, which is my style.

I consider the OE type M400 bearings to be the best in the business, so I installed new M400 bearings purchased from a Chevy dealer when I rebuilt the engine.

But, again, this type of bearing damage is usually due to something other than excess loading - tight clerance due to dimensional non-conformities of the main bearing saddles big end rod bores, cranshaft, or excessively tight clearance at assembly - oil starvation or debris in the oil.

Even though trimetal bearings don't have the load capcity or fatigue resistance of M400s I don't think this damage would be caused by overlaoding or fatigue. With proper dimensions/clearances and an uninterrupted supply of clean oil, bearing failure from overloading or fatigue is very rare. In such cases I would check dimensions and the oiling system until I found a suspect cause of this damage and, of course, correct the cause before the engine is reassembled.

For every effect there is a cause.

One should always do a "forensic teardown" looking for excess wear or any possible problems, especially if operational problems were noted prior to teardown. Otherwise, an internal problem can go overlooked, which will eventually cause trouble and probably shorten engilne life.

Duke

[Joe L.]

Re: Mystery Bearings???

Chuck-----

That's how it does appear, but I don't think that's how it is. In other words, I don't think that there was any manufacturing deficiency with respect to these bearings. Rather, I think that the peculiar "exposed copper line" is a characteristic of the normal manufacturing of these, particular, bearings AND the excessive wear. These tri-metal bearings are not necessarily manufactured with a completely "even" layer across the width of the bearing for each of the 3 layers. The lower, thick copper-lead alloy layer may have a slight ridge in the center in order to laterally "lock in" the subsequent layers. Consequently, when excessive wear on the upper 2 layers occurs, the "center ridge" of the copper-lead alloy layer is first to appear.

[Chuck S.]

Re: Mystery Bearings???

Interesting...almost a "wear bar" for bearings. That does make sense. I'm surprised that the "copper line" extends so far around the shell.

Considering this center ridge, then the wear appears to be pretty uniform across the bearing shell. My engine had similar "mystery bearings", only there wasn't much "mystery" involved...large random wear patches were showing copper. I don't recall seeing any "wear bars", but I didn't take pictures, prfering to forget the image.

[Clem Z.]

looks like a hunk of debris came thru the oil

hole in the crank and got picked up by the bearing. if you want to makes sure this never happens on a new rebuild after cleaning all the oil gallerys well with special brushes plug the oil filter bypass valve in the oil filter adapter. any time you start a engine till the oil pressure equalizes on both side of the micarta disk in the adapter unfiltered oil goes thru the engine.any time you have crank work done you MUST clean the oil passages with special brushes made for this purpose.

[Dick W.]

Re: looks like a hunk of debris came thru the oil

And plenty of warm soapy water